Robust ingot for the production of components made of metallic solid glasses

ABSTRACT

A method for production of an ingot of a bulk glass-forming alloy, comprising the steps of: Providing a homogeneous melt of a bulk glass-forming alloy; casting the homogeneous melt into a casting mould, whereby the casting mould does not cool down below the glass-transition temperature of the alloy at the contact surface to the melt for at least 5 seconds; and cooling down the melt below the glass transition temperature of the bulk glass-forming alloy while obtaining the ingot.

The invention relates to a method for the production of mechanically and thermally stable ingots (also called preform) made of alloys that can form a bulk metallic glass. The invention also relates to an ingot of a bulk glass-forming alloy that is produced with the method according to the invention, and to the use of said ingot in a casting process.

Discovered some 50 years ago at the California Institute of Technology, metallic glasses have since been the subject of comprehensive research. Over the years, it has been possible to continuously improve the processability and properties of this class of materials. While the first metallic glasses were simple binary alloys (made up of two components) whose production required cooling rates in the range of 10⁶ Kelvin per second (K/s), more recent and more complex alloys can be transferred to the vitreous state at considerably lower cooling rates in the range of several K/s. This has a significant effect on process control and the components that can be realised. The cooling rate from which the melt no longer crystallises, but rather solidifies in the form of a glass, is called the critical cooling rate. It is a system-specific parameter that depends strongly on the composition of the melt and, in addition, defines the maximum component thickness that can be attained. Considering that the heat energy stored in the melt needs to be dissipated by the system at a sufficiently rapid rate, it becomes clear that only components of low thickness can be produced using systems with high critical cooling rates. For this reason, metallic glasses were initially produced according to the melt spinning process (German: Schmelzspinnverfahren). In this process, the melt is stripped onto a rotating copper wheel and solidifies in glass-like manner in the form of thin bands and/or sheets with thickness values in the range of several hundredths to tenths of a millimetre. Due to the development of new, more complex alloys with considerably lower critical cooling rates, other production processes can be utilised to an increasing degree. Today's glass-forming metallic alloys can be transferred to the vitreous state by simply casting a melt into cooled copper dies. The component thicknesses that can be realised in this context are alloy-specific in the range of several millimetres to centimetres. Alloys of this type are called bulk metallic glasses (BMG). In the scope of the present invention, a bulk metallic glass shall be understood to be a material with a critical casting thickness of at least one millimetre. A large number of said alloy systems are known nowadays. They are usually classified according to their composition, with the alloy element with the highest weight fraction being called the base element. Currently existing systems comprise, inter alia, precious metal-based alloys, such as, for example, gold-, platinum-, and palladium-based bulk metallic glasses, early transition metal-based alloys, such as, e.g., titanium or zirconium-based bulk metallic glasses, late transition metal-based systems, e.g. based on copper, nickel or iron, but also systems based on rare earth metals, e.g. neodymium or terbium.

Compared to classical crystalline metals, bulk metallic glasses typically comprise at least one of the following properties:

-   -   higher specific strength which allows, for example, for thinner         wall thicknesses;     -   higher hardness which allows the surfaces to be particularly         scratch-resistant;     -   much higher elastic elasticity and resiliency;     -   thermoplastic deformability; and     -   higher corrosion resistance.

Components made of bulk metallic glasses can be produced by means of casting processes, since the cooling rates required for amorphous solidification can be attained in said processes. In order to obtain an amorphous component from a bulk metallic glass, it is usually necessary to rapidly transfer the melt of a bulk glass-forming alloy to a casting mould. Preferably, this is achieved by filling the melt into the casting mould by means of injection (injection moulding) or suction (suction casting). High cooling rates can be attained by this means and three-dimensional components can be produced from bulk metallic glasses. The use of casting processes, such as, e.g., injection moulding, allows low manufacturing tolerances to be attained.

Casting processes require ingots of the alloy to be processed that serve as a reservoir of the material to be processed and can be molten homogeneously. For this purpose, the ingots must have a sufficient volume such that sufficient material for the entire cast component is available and additional spaces of the casting mould (e.g., the sprue) can also be filled. It is therefore desirable to have ingots of the largest possible size.

For production of ingots from bulk glass-forming alloys, a homogeneous bulk glass-forming alloy is produced first. For this purpose, the individual components are mixed together and heated beyond the melting point such that a homogeneous alloy is produced. The individual components can be melted, for example, in an electric arc or by means of inductive heating. The homogeneous alloy is then filled into casting moulds and chilled such that an ingot is produced. In general, these ingots take the shape of cylindrical rods. For the ingots to contain sufficient material in order to completely fill the casting mould for a casting process for a three-dimensional component, the ingots must be dimensioned sufficiently large. Typical diameters of cylindrical ingots made of bulk glass-forming alloys are in the range of approximately 20 mm. The length of an ingot is preferred to be at least 3 cm.

Processes are already known from U.S. Pat. No. 5,279,349, in which amorphous moulded parts can be obtained through the use of preheated casting moulds. Here, the melt is exposed to pressure during the cooling process. By means of said processes, very small, amorphous ingots can be produced since the moulded part must not exceed the critical casting thickness in any dimension. However, due to their limited size, said completely amorphous ingots can provide only a very limited amount of material for a casting process. It is another disadvantage of amorphous ingots in the use of casting processes that they can be molten only slowly due to their comparably poor thermal conductivity.

The production of high-quality ingots made of materials with a high critical casting thickness and dimensions exceeding the critical casting thickness is difficult. Firstly, there is a significant amount of scrap material during production, since known ingots often crack as early as during the production process. Moreover, some of the ingots produced by conventional technique crack during transport or during the heating in the course of the actual step of producing a three-dimensional component by means of casting processes. The cracking of ingots during the production of a three-dimensional component is disadvantageous since the cracks formed interrupt the conduction of heat. This increases the process duration of the production of three-dimensional components. In order to prevent conventional ingots that survived the production process without damage from cracking, the ingot needs to be heated very slowly to the melting temperature. Typically, the melting of the ingots takes at least 80 seconds.

OBJECT OF THE INVENTION

One object of the present invention was to provide an ingot made of a bulk glass-forming alloy with high critical casting thickness that does not crack during the production process and can be heated up more rapidly during the thermal processing, such as, e.g., injection moulding.

Moreover, it was an object of the invention to provide a method for production of an ingot made of a bulk glass-forming alloy with high critical casting thickness that does not crack during the production process.

Another object of the invention was to provide ingots made of bulk glass-forming alloys that can be heated up more rapidly than conventional ingots.

A contribution to solving at least one of the aforementioned objects is made by the subject matters of the independent claims.

A first aspect of the invention relates to a method for production of an ingot (20) of a bulk glass-forming alloy, comprising the steps of:

-   -   a. Providing a homogeneous melt (10) of a bulk glass-forming         alloy;     -   b. casting the homogeneous melt into a casting mould, whereby         the casting mould does not cool down below the glass-transition         temperature of the alloy at the contact surface to the melt for         at least 5 seconds; and     -   c. cooling down the melt below the glass transition temperature         of the bulk glass-forming alloy while obtaining the ingot (20).

The composition of the bulk glass-forming alloy is not limited in any way. Preferably, a bulk glass-forming alloy shall be understood to be an alloy with a critical casting thickness of at least one millimetre. This means that said alloy, at a suitable cooling rate, can solidify amorphously up to a thickness of one millimetre.

Bulk glass-forming alloys shall be understood to be alloys that can comprise, under certain thermal conditions, metallic binding characteristics in the solid-state and, simultaneously, can comprise an amorphous, i.e. non-crystalline, phase. The alloy can be based on different elements. In this context, “based” shall be understood to mean that the respective element represents the largest fraction, relative to the weight of the alloy. Components, which can preferably also be the basis of the alloy, can, for example, be selected from:

-   -   A. Metals from groups IA and IIA of the periodic system of the         elements, e.g. magnesium, calcium;     -   B. metals from groups IIIA and IVA, e.g. aluminium or gallium;     -   C. early transition metals from groups IVB to VIIIB, such as,         e.g., titanium, zirconium, hafnium, niobium, tantalum, chromium,         molybdenum, manganese;     -   D. late transition metals from groups VIIIB, IB, IIB, such as,         e.g., iron, cobalt, nickel, copper, palladium, platinum, gold,         silver, zinc;     -   E. rare earth metals, such as, e.g., scandium, yttrium, terbium,         lanthanum, cerium, neodymium. Gadolinium and     -   F. non-metals, such as, e.g., boron, carbon, phosphorus,         silicon, germanium, sulfur

Preferred combinations of elements in bulk metallic glasses are selected from:

-   -   Late transition metals and non-metals, whereby the late         transition metal is the basis, for example Ni—P, Pd—Si,         Au—Si—Ge, Pd—Ni—Cu—P, Fe—Cr—Mo—P—C—B;     -   early and late transition metals, whereby either metal can be         the basis, such as, e.g., Zr—Cu, Zr—Ni, Ti—Ni, Zr—Cu—Ni—Al,         Zr—Ti—Cu—Ni—Be;     -   metals from group B and rare earth metals, whereby metal B is         the basis, such as, e.g., Al—La, Al—Ce, Al—La—Ni—Co,         La—(Al/Ga)—Cu—Ni; and     -   metals from group A and late transition metals, whereby metal A         is the basis, such as, e.g., Mg—Cu, Ca—Mg—Zn, Ca—Mg—Cu

Further, particularly preferred examples of alloys that can form bulk metallic glasses are selected from the group consisting of Ni—Nb—Sn, Co—Fe—Ta—B, Ca—Mg—Ag—Cu, Co—Fe—B—Si—Nb, Fe—Ga—(Cr,Mo)(P,C,B), Ti—Ni—Cu—Sn, Fe—Co-Ln-B, Co—(Al,Ga)—(P,B,Si), Fe—B—Si—Nb, and Ni—(Nb,Ta)—Zr—Ti. In particular, the bulk metallic glass can be a Zr—Cu—Al—Nb alloy. Preferably, said Zr—Cu—Al—Nb comprises in addition, other than zirconium, 23.5-24.5% by weight copper, 3.5-4.0% by weight aluminium as well as 1.5-2.0% by weight niobium, whereby the weight fractions add up to 100% by weight. The latter alloy is commercially available from Heraeus Deutschland GmbH by the name of AMZ4®. In a further particularly preferred embodiment, the bulk glass-forming alloy can contain or consist of the elements, zirconium, titanium, copper, nickel, and aluminium. Particularly stable ingots can be produced from bulk glass-forming alloys having said composition. An alloy that is particularly well-suited for production of stable ingots comprises the composition Zr_(52.5)Ti₅Cu_(17.9)Ni_(14.6)Al₁₀, whereby the indices specify the mol-% of the respective element in the alloy.

Due to the intrinsic heat conduction of the material, even at maximum attainable cooling rate there is a maximum casting thickness, whereby the casting needs to be smaller than that in at least one dimension in order to still be able to form a homogeneous amorphous phase. Preferably, the bulk glass-forming alloy comprises a critical casting thickness of at least 5 mm, in particular of at least 7 mm, and particularly preferably of at least 10 mm. In the scope of the present invention, the critical casting thickness (maximum casting thickness) is a measure of how easy or difficult it is to transfer a metallic alloy to the vitreous state.

In order to determine the critical casting thickness in the scope of the invention, the alloy to be tested is processed in an electric arc to form a homogeneous melt and is subsequently poured into a water-cooled copper casting mould (also called a die). The mass of the copper die is preferred to be larger by at least a factor of 7 than the mass of the melt of the alloy to be tested that is being filled into it. The temperature of the homogeneous melt before casting is preferred to be at least 200° C., in particular 300° C., and particularly preferably at least 400° C. above the melting temperature. The temperature of the copper die is 20° C. For determination of the critical casting thickness, cylindrical moulded parts with increasing diameters differing by 1 mm are being cast (e.g. 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, etc.). The cylindrical moulded parts thus generated are tested for their crystalline fraction by means of dynamic differential scanning calorimetry (DSC). The critical casting thickness reported is the cylinder diameter that is 1 mm smaller than the cylinder diameter at which the formation of a crystalline phase is first measured by means of DSC. The determination of the presence of a crystalline phase was measured by DSC procedure 2) as described herein.

In step a) of the present invention, a homogeneous melt of a bulk glass-forming alloy is provided. The provision of the homogeneous melt is done preferably by melting and combining the individual elements of the alloy. The melting of the individual elements preferably takes place in an electric arc or by means of inductive heating. The temperature of the homogeneous melt is preferred to be at least 200° C., in particular at least 300° C., and particularly preferably at least 400° C. above the melting temperature of the respective bulk glass-forming alloy. In a preferred embodiment, the temperature of the melt, measured in degrees centigrade, is at least 20%, in particular at least 50%, above the melting temperature of the alloy, since this allows particularly stable ingots to be produced.

The casting of the homogeneous melt into a casting mould takes place in step b). The shape of the casting mould is not limited in any way according to the invention. Preferably, the casting mould is cylindrical in shape. Preferably, the volume of the casting mould to be filled is dimensioned to be larger in all three directions of space than the critical casting thickness of the bulk glass-forming alloy. The material of the casting mould can preferably be selected from steel, titanium, copper, ceramics or graphite. Preferably, the casting mould comprises a device by means of which the casting mould can be actively heated and/or cooled. In an embodiment of the invention, the casting mould can be actively heated, e.g. by electrical heating.

The ratio of the weight of the casting mould and the weight of the melt is preferred to be in the range of 7:1 or more, particularly preferably in the range of 10:1 or more. In a preferred embodiment of the invention, the casting mould can be coated in the region that contacts the melt. The material of said coating of the casting mould is preferably selected from the group consisting of boron nitride, aluminium oxide (e.g. Al₂O₃), and yttrium oxide (e.g. Y₂O₃). Preferably, the coating comprises or consists of a powder. The thickness of the coating, in particular of the powder coating, can be in the range of 10-50 μm in an embodiment. A powder layer can have advantageous effects on the mechanical properties of the ingot to be produced. The coating can serve, inter alia, for allowing the ingot to be removed more easily from the casting mould.

According to the invention, the casting mould does not cool down below the glass-transition temperature of the bulk glass-forming alloy at the contact surface to the melt for at least 5 seconds, in particular for at least 10 seconds, and particularly preferably for at least 30 seconds. In the scope of the invention, the term “melt” is used even after the liquid melt was transferred into the casting mould even if the solidification process has commenced already and the bulk glass-forming alloy has partially or completely solidified as long as the temperature has not yet dropped below the glass transition temperature.

In a preferred embodiment of the invention, no site of the contact surface of the casting mould to the melt cools below the glass-transition temperature of the bulk glass-forming alloy for the specified period of time. The determination of the glass-transition temperature of the alloy is described in “Methods”. In a preferred embodiment of the invention, the temperature of the casting mould at the contact surface to the melt is at least 10° C., in particular at least 20° C., and particularly preferably at least 40° C. or at least 80° C. above the glass-transition temperature of the bulk glass-forming alloy for the period of time specified above.

For measurement of the temperature of the casting mould at the contact surface, a temperature measuring probe can be appropriately incorporated into the casting mould such that it extends to the contact surface of the casting mould to the melt and performs the measurement at this site. The temperature measurement preferably takes place at the site equal to half of the length of the longest extension of the ingot. Preferably, the temperature of the casting mould before filling it with the melt is set appropriately such that the temperature, after casting, of the casting mould at the contact site to the melt does not drop below the glass-transition temperature of the alloy for at least 5 seconds, in particular for at least 10 seconds, and particularly preferably for at least 30 seconds after contact with the casting mould.

Preferably, the casting mould is heated prior to contact with the melt. The preferably set temperature of the casting mould right before casting the melt is at least 250° C., in particular at least 400° C., and particularly preferably at least 500° C. The casting mould can be heated, for example, in a furnace. Alternatively, the casting mould can be actively heated, e.g. by electrical heating.

Preferably, no additional pressure significantly above standard atmospheric pressure is exerted on the melt after casting the melt. “Significantly above standard atmospheric pressure” shall be understood to mean an overpressure of 1 bar or more in the scope of the invention.

In step c), the melt is cooled down below the glass transition temperature of the bulk glass-forming alloy while obtaining the ingot (20). Preferably, the melt is cooled down to room temperature. The cooling rate in step c) is not limited in any way according to the invention. In a feasible embodiment, the melt is allowed to cool down to room temperature without any additional intervention (heating and/or cooling). Alternatively, the melt can be actively cooled below the glass transition temperature in order to accelerate the process.

The method according to the invention allows an ingot that does not crack during the production process to be produced from a bulk glass-forming alloy. Moreover, the method allows an ingot to be produced that does not crack when it is heated to the melting temperature of the alloy within a period of up to 50 seconds. In particular, an ingot can be produced that does not crack when it drops three times onto a level horizontal steel surface from a height of 30 cm. In particular, the method allows an ingot to be produced that does not comprise an amorphous layer at the surface. The absence of an amorphous layer can be determined in a light microscope.

Ingot

A further aspect of the invention relates to an ingot of a bulk glass-forming alloy, whereby the alloy has a critical casting thickness of at least 5 mm and whereby the ingot has an extension in all three directions of space that is larger than the critical casting thickness, characterised in that the ingot comprises a crystalline fraction of at least 90% by weight, in particular at least 95% by weight, and particularly preferably at least 98% by weight, as measured by means of DSC.

Preferably, the critical casting thickness of the alloy is at least 7 mm and, in particular, at least 10 mm. The ingot according to the invention can be produced by means of the method described herein. In a preferred embodiment, the ingot according to the invention comprises no amorphous layer on the surface. In the scope of the present invention, the term “no amorphous layer” shall be understood to mean a layer that is no thicker than 200 μm, in particular no thicker than 100 μm, and particularly preferably no thicker than 50 μm. The absence of an amorphous layer can preferably lead to the reduction of internal stress in the ingot. The absence of an amorphous layer on the surface of the ingot can be determined by means of light microscopy (reflected light microscope). For this purpose, a cross-section of the ingot is produced by means of a diamond saw. The cross-section is also called metallurgical micrograph or transverse section. The absence of amorphous fractions can be determined by the absence of a phase transition that is visible to the eye in the light microscope. Phase transitions can be identified in the light microscope as transitions of different colour or different contrast. Reference shall be made to FIGS. 1 to 3 in this context. FIG. 1 shows a micrograph of a cross-section through an ingot that comprises amorphous regions. Said amorphous regions can be detected as bright regions towards the edge (arrow 1). The internal region of the tested ingot comprises no bright regions (arrow 2). In contrast, FIG. 2 shows a micrograph of a cross-section through an ingot that comprises no amorphous regions. This can be recognised by the uniform appearance of the material without any bright patches. FIG. 3 shows a metallurgical micrograph of the sample from FIG. 2 at higher magnification. The polycrystalline structures and/or their grain boundaries are clearly evident therein. It is also evident that the crystalline structure of the ingot according to the invention extends all the way to the edge, which confirms the absence of an amorphous phase (e.g. in the circled region). If there was an amorphous phase, it would preferably be formed at the edge first, since the cooling rates can potentially be the highest in this location.

In an embodiment, the total volume of the amorphous layer on the ingot can be 5% or less, in particular 3% or less. The crystallinity of the ingot can be measured by means of Differential Scanning calorimetry (DSC). Preferably, the ingot is solid and comprises no hollow spaces, such as, e.g., air inclusions. The shape of the ingot is not limited in any way according to the invention. The ingot can comprise a cylindrical shape in an embodiment. Preferably, the numerical value of the cylinder diameter is at least 5 mm, in particular at least 15 mm, and particularly preferably at least 25 mm, each subject to the condition of the diameter being larger than the critical casting thickness of the bulk glass-forming alloy. The length of the cylinder is preferred to be at least 3 cm.

A further aspect of the invention relates to a method for production of three-dimensional components from bulk metallic glasses by means of casting processes, in particular injection moulding, through the use of the inventive ingot of a bulk glass-forming alloy.

During the production of the three-dimensional component by means of casting processes, such as, e.g., injection moulding, the ingot according to the invention is being melted to produce a homogeneous melt (30). Preferably, the complete melting of the ingot (20) takes no longer than 60 seconds, in particular no longer than 40 seconds, and particularly preferably no longer than 20 seconds, whereby the ingot can be heated without cracking.

Typically, conventional ingots can be melted only clearly more slowly as they would crack otherwise. This is associated with the drawbacks described above. Typically, the heating time of known ingots of the same dimensions is in the range of 80 seconds. After melting the ingot (20) the homogeneous melt (30) is cast, in particular injected, into the casting mould for a three-dimensional component (40). Preferably, the casting mould for production of the three-dimensional component by means of casting processes is dimensioned appropriately such that it exceeds at no site the critical casting thickness of the alloy used in the process as it allows completely amorphous three-dimensional components to be produced. In particular, the ingot can be used for production of three-dimensional components that can be produced at high throughput in an injection moulding machine.

Measuring Methods

X-Ray Diffractometry (XRD)

The XRD measurements are performed in accordance with DIN EN 13925-1:2003-07 and DIN EN 13925-2:2003-07. A diamond saw is used to produce a transverse section of the material to be tested. The planar surface of the transverse section is in the range of approximately 1 cm². The measuring details used generally herein can be summarised as follows: Diffraction: Bragg-Brentano; Detector: Scintillation Counter; Radiation: Cu_(Kα) 1.5406 Å; Source: 40 kV, 25 mA; Measuring method: Reflection.

The empty sample holder is measured as an internal reference first in order to determine the background signal. Said background measurement is subtracted from all subsequent measurements of the samples to be tested.

Discrete diffraction signals in the diffractogram, if any, can be analysed according to the Debye-Scherrer method using the Bragg equation. If there are any discrete crystalline peaks visible above the statistical noise, the crystalline fraction is presumed to be at least 5% by weight. If the diffractogram shows no evidence of sharp diffraction signals, the crystalline fraction is less than 5%.

DSC: Measurement

The DSC measurements in the scope of the invention are performed in accordance with DIN EN ISO 11357-1:2017-02 and DIN EN ISO 11357-3:2018-07. The sample to be measured, in the form of a thin disc or sheet (approximately 80-100 mg), is placed in the measuring device (NETZSCH DSC 404F1, NETZSCH GmbH, Germany). The heating rate is 20.0 K/min. Al₂O₃ is used as the crucible material. The heat flow is measured relative to an empty reference crucible such that only the thermal behaviour of the sample is measured.

The measuring procedure takes place according to the following steps:

-   -   a) The sample to be measured is heated at the heating rate         specified above to a temperature T just below the melting         temperature (T=0.75*Tm) and the heat flow is measured. The         measurement is completed once no heat flow in connection to         phase transitions can be measured any longer. In particular, the         measurement is stopped once an exothermic signal in connection         with the crystallisation process is detected completely. In the         examples included herein, the measurement takes place, e.g.,         from room temperature to approximately 600° C.     -   b) The sample is allowed to cool down to room temperature.     -   c) The sample is heated again at the same heating rate to the         same temperature as in step a) and the heat flow is measured.     -   d) The measurement from step c) is subtracted from the         measurement from step a) obtaining the difference between the         measurements. The difference between the measurements is used to         determine the crystallisation enthalpy, if any, by forming the         integral.

1) Measurements on Samples with a Small Amorphous Fraction (e.g. Ingot According to the Invention)

Samples expected to be predominantly crystalline and to comprise only a small fraction of amorphous phase are measured according to the measuring method specified above. The sample, e.g. from an ingot according to the invention, is heated to a temperature of T=0.75*Tm (75% of the melting temperature (Tm) in ° C.) in step a). If, after subtracting the reference measurement from step c), no heat flow can be determined near the crystallisation temperature, the sample is presumed to be completely crystalline (measuring inaccuracy 5%). The complete crystallinity of the sample after passage through the measuring method can be confirmed additionally by means of XRD by the absence of broad unspecific signals in the diffraction diagram that would indicate an amorphous phase. The amorphous fraction of samples with more than 5% by weight can be determined by comparison of the crystallisation enthalpy of the unknown sample to the numerical value of the completely amorphous sample from DSC procedure 2) (see below).

2) Determination of the Critical Casting Thickness

A sample of each of the cast cylinders is measured by means of DSC in order to determine the critical casting thickness. As long as the diameter of the cylinders is below the critical casting thickness, the sample is completely amorphous before commencement of the measurement and crystallises during the DSC measurement in step a) of the measuring method. The crystallisation enthalpy of the alloy is determined from the measurement of the completely amorphous material. The crystallisation enthalpy is determined for all samples of increasing cylinder diameter. The crystallisation enthalpy determined for samples with a cylinder diameter below the critical casting thickness is constant within the limits of the measuring inaccuracy. As soon as the cylinder diameter exceeds the critical casting thickness, a smaller crystallisation enthalpy value is measured in the DSC measurement than for the smaller diameters, since part of the material has already crystallised and this does no longer take place during the DSC measurement. The critical casting thickness is determined as the cylinder diameter up to which the crystallisation enthalpy, for increasing diameters, is constant.

3) Glass Transition Temperature

In the scope of the present invention, the glass transition temperature is measured according to ASTM E1365-03 as follows. The sample to be tested is placed in a crucible in a DSC device (NETZSCH DSC 404F1, NETZSCH GmbH, Germany). The system is heated and cooled according to the following scheme and the respective heat flow is measured in steps a) and c).

-   -   a) Heating to a temperature of 0.75*Tm at a heating rate of 20         K/min.     -   b) Cooling to room temperature     -   c) Heating to the same temperature as in step a) at the same         heating rate; and     -   d) Cooling to room temperature.

The result obtained in the experiment is the enthalpy of the sample as a function of the temperature. The crystallisation of the amorphous sample takes place in step a). The thermal behaviour of the already fully crystallised sample is recorded in step c).

In order to determine the glass transition temperature, the measurement from step c) is subtracted from the measurement from step a). The resulting curve contains an endothermic transition at lower temperature and an exothermic signal at higher temperature. The signal at higher temperature corresponds to the crystallisation process. The endothermic signal corresponds to the glass transition. In order to determine the glass transition temperature, a tangent line to the base line is determined (by linear fit) before the glass transition range. A second tangent is determined in the inflection point (corresponding to the peak value of the first time derivative) of the glass transition range. The temperature value at the intersection of the two tangents indicates the glass transition temperature (T_(f) according to AST; 1356-03).

EXAMPLES

The individual components were melted in an inert gas by means of inductive melting to form a homogeneous alloy with a composition of Zr_(52.5)Ti₅Cu_(17.9)Ni_(14.6) Al₁₀. Said alloy has a glass transition temperature of 403° C. A total of 80 g of the homogeneous alloy were heated by means of inductive heating in a melting crucible to a temperature above the melting temperature of the alloy (805° C.). The temperatures of the respective melts in each experiment are shown in Table 1. The casting mould was heated in a furnace to a temperature defined in Table 1 for each case. Subsequently, the respective homogeneous melt according to Table 1 was filled into a casting mould. The casting mould was cylindrical in shape and had an internal diameter of 19 mm. The temperature of the melt was measured continuously after filling it into the cylindrical casting mould. The measuring values of the temperature of the melt after 10 seconds in the casting mould are given in Table 1 for each case.

TABLE 1 Example 1 2 3 4 5 T_(melt)[° C.] 1050 1100 1200  1250  1350  T_(casting mould)[° C.]  50  50 250 400 600 Casting mould Copper Steel Steel Steel Steel Weight ratio 1:17 1:15 1:9 1:15 1:15 Coating of the casting none BN Y₂O₃ BN Al₂O₃ mould T_(casting mould) after 10 s  150  150 410 420 approx. [° C.] 550 Ingot quality poor poor good good very good

Examples 1 and 2 in Table 1 are reference examples, Examples 3-5 are examples according to the invention. The quality of the cast ingots was assessed according to the following criteria: Cast parts of poor quality crack already while they cool down in the casting mould. Cast ingots of good quality stay intact when they are heated to the melting temperature at a power of 5 kW for at most 50 seconds. Ingots of very good quality additionally survive a drop test from a height of 30 cm onto a level steel plate, performed thrice, without cracking. It is evident from examples 1-5 that ingots, whose melt temperature after 10 seconds was above the glass transition temperature, were clearly more robust than ingots whose melt temperature was below that.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a light microscope image showing the cross-section of an ingot that was produced according to Example 1 as a reference experiment. The bright regions in the cross-section, indicated by arrows in exemplary manner, show amorphous regions (arrow 1) surrounded by darker crystalline regions (arrow 2). Moreover, it is evident from FIG. 1 that the ingot is cracked.

FIG. 2 shows a light microscope image showing the cross-section of an ingot that was produced according to Example 4. The cross-section of an ingot according to Example 4 shows a homogeneous distribution of material without bright regions that would indicate amorphous phases.

FIG. 3 shows a magnification of the sample according to the invention from FIG. 2. The image shows the multi-crystalline structure of the ingot all the way into the marginal region of the cross-section.

FIG. 4 shows a schematic depiction of the process flow from the individual components of the bulk glass-forming alloy (5) to the component made of bulk metallic glass (40). The process flow goes through the following steps: Individual components of the bulk glass-forming alloy (5), homogeneous melt (10), ingot made of bulk glass-forming alloy (20), homogeneous melt of the bulk glass-forming alloy (30), and component made of bulk metallic glass (40). 

1. A method for production of an ingot of a bulk glass-forming alloy, comprising the steps of: a. Providing a homogeneous melt of a bulk glass-forming alloy; b. casting the homogeneous melt into a casting mould, whereby the casting mould does not cool down below the glass-transition temperature of the alloy at the contact surface to the melt for at least 5 seconds; and c. cooling down the melt below the glass transition temperature of the bulk glass-forming alloy while obtaining the ingot.
 2. The method according to claim 1, whereby the casting mould does not cool down below the glass-transition temperature of the alloy at the contact surface to the melt for at least 10 second.
 3. The method according to claim 1, whereby the bulk glass-forming alloy has a critical casting thickness of 5 mm or more.
 4. The method according to claim 1, whereby the size of the ingot in the three directions of space is larger than the critical casting thickness.
 5. The method according to claim 1, whereby the ingot comprises a crystalline fraction of at least 90%, relative to the weight, as measured by means of DSC.
 6. The method according to claim 1, whereby the ingot comprises a crystalline fraction of at least 95%, relative to the weight, as measured by means of DSC.
 7. The method according to claim 1, whereby the casting mould is coated with a material selected from the group consisting of boron nitride, Y₂O₃, and aluminium oxide.
 8. The method according to claim 1, whereby the ratio of the weight of the melt and the weight of the casting mould is 1:7 or less.
 9. The method according to claim 1, whereby the temperature of the melt in step a) is at least 20% above the melting temperature, as measured in degrees centigrade.
 10. An ingot of a bulk glass-forming alloy, comprising a critical casting thickness of at least 5 mm, whereby the ingot has an extension in at least three directions of space that is larger than the critical casting thickness, characterised in that the ingot comprises a crystalline fraction of at least 90% by weight, as measured by means of DSC.
 11. A method for production of a three-dimensional component from a bulk metallic glass by means of casting processes, characterised in that an ingot (20) according to claim 10 is being melted for the casting process.
 12. The method according to claim 11, whereby the melting of the ingot takes no longer than 60 seconds, in particular no longer than 40 seconds. 